1. Field of the Invention
The present invention relates to a multibeam type optical disc read apparatus, and more particularly to a multibeam type optical disc read apparatus in which different light beams are applied at the same time to a plurality of adjacent tracks of an optical disc, such CD-ROM, CD-WO, DVD, DVD-ROM, DVD-RAM, MO, and LD and data recorded on the tracks applied with the light beams are read with a record data read system in accordance with a detected output of each reflected light beam.
2. Description of the Related Art
A multibeam method is one of the methods of reading record data from a CD-ROM at high speed. With this method, different light beams are applied at the same time to a plurality of adjacent tracks of an optical disc formed with a spiral track, data recorded on the tracks applied with the light beams are read with a record data read system for each track in accordance with a detected output of each reflected light beam, and the read data is sequentially output in the record order by preventing the read data from being duplicated or omitted.
Each of a plurality of light beams of such a multibeam optical disc read apparatus is required to be correctly focussed onto an optical disc signal plane. If not, record data cannot be read with a light beam in an out of focus state. If a focussing optical system is provided for each of a plurality of light beams, it is possible to focus all the light beams onto the optical disc signal plane by independently controlling these focussing optical systems. However, provision of a plurality of focussing optical systems is not practical from both the technical and economical viewpoints. From this reason, a single focussing optical system is generally used. An optical system is associated with aberration. Even if the center of an optical axis is in an in-focus state, a point spaced apart from the center of the optical axis is in an out of focus state. In order to solve this problem, a plurality of light sources for generating a plurality of light beams are disposed in line and adjusted so as to make a light source spaced apart from the optical axis be shifted toward the optical axis.
However, such adjustment is not easy and it is not preferable to use a plurality of light sources (laser diodes) in order to realize a cost effective system. A a plurality of light beams are focussed on a signal plane of an optical disc, the focus states of the light beams are different. Therefore, in order to compensate for the high frequency component attenuation to be caused by the spatial frequency characteristics (MTF) of a detection output of each light beam, a waveshape equalizer circuit is used for raising a gain of the high frequency components of the detection output and suppressing inter-code interference. It is not economical that a plurality of different waveshape equalizer circuits are used for a plurality of light beams.
In a multibeam optical disc read apparatus of this invention, an optical system including an objective lens applies n light beams at the same time to respective n tracks on a signal plane of an optical disc in an in-focus state near at the signal plane, and signal processing means generates an RF signal for each light beam from a detection output of the light beam reflected from the signal plane, and reads data recorded on the tracks to which the light beams were applied, basing upon the detection output of the light beam, wherein the optical system is set so that the signal plane of the optical disc is positioned-at the middle of respective in-focus points of the n light beams as viewed along an optical axis direction of the objective lens.
It is therefore possible to avoid a poor in-focus state of each of the n light beams relative to the signal plane and to reliably read record data from an optical disc.
According to an embodiment of the invention, grading means diffracts a light beam generated from a single laser diode light source and generates 0 order, +/xe2x88x92 first order, . . . , +/xe2x88x92 m-th order diffraction light beams. The i-th order diffraction light beams are symmetrical relative to the 0 order diffraction light beam. Therefore, the focus states of the i-th order diffraction light beams are identical and the spatial frequency characteristics are also identical, so that the waveshape equalizer circuits of the same characteristics can be used.
According to an embodiment of the invention, first and second focus control operations are executed by first determining a control amount necessary for focussing the 0 order diffraction light beam onto the signal plane of an optical disc, and then modifying the control amount so that the signal plane of the optical disc is positioned at the middle of the in-focus points of the 0 order, +/xe2x88x92 first order, . . . , +/xe2x88x92 m-th order diffraction light beams.